This is the author’s version of a work that was submitted/accepted for pub- lication in the following source:
Frost, Ray L.&Zbik, Marek(2010) Influence of smectite suspension struc- ture on sheet orientation in dry sediments : XRD and AFM applications.
Journal of Colloid and Interface Science, 346(4), pp. 311-316.
This file was downloaded from: http://eprints.qut.edu.au/32137/
Notice: Changes introduced as a result of publishing processes such as copy-editing and formatting may not be reflected in this document. For a definitive version of this work, please refer to the published source:
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Influence of smectite suspension structure on sheet orientation in dry sediments: XRD and AFM applications
Marek S. Zbik and Ray L. Frost ••••
Faculty of Sciences, Queensland University of Technology, 2 George Street, GPO Box 2434, Brisbane Qld 4001 Australia.
Abstract
The structure-building phenomena within clay aggregates are governed by forces acting between clay particles. Measurements of such forces are important to understand in order to manipulate the aggregate structure for applications such as dewatering of mineral processing tailings. A parallel particle orientation is required when conducting XRD investigation on the oriented samples and conduct force measurements acting between basal planes of clay mineral platelets using atomic force microscopy (AFM). To investigate how smectite clay platelets were oriented in silicon wafer substrate from suspension and its effect range of method like SEM, XRD and AFM were employed. From these investigations, we conclude that high clay concentrations and larger particle diameters (up to 5 µm) in suspension result in random orientation of platelets in the substrate. The best possible laminar orientation in the clay dry film, represented in the XRD 001/020 intensity ratio of 47 was obtained by drying thin layers from 0.02 wt% clay suspensions of the natural pH.
Conducted AFM investigations show that smectite studied in water based electrolytes show very long range repulsive forces lower in strength than electrostatic forces from double layer repulsion. It was suggested that these forces may have structural nature. Smectite surface layers rehydrate in water environment forms surface gel with spongy and cellular texture which cushion approaching AFM probe.
This structural effect can be measured in distances larger than 1000 nm from substrate surface and when probe penetrate this gel layer, structural linkages are forming between substrate and clay covered probe. These linkages prevent subsequently smooth detachments of AFM probe on way back when retrieval. This effect of tearing
• Author to whom correspondence should be addressed ([email protected])
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new formed structure apart involves larger adhesion-like forces measured in retrieval.
It is also suggested that these effect may be enhanced by the nano-clay particles interaction.
Keywords: smectite suspension, nano-colloids, nano-clays, AFM, suspension structure.
3 Introduction
Kaolinite, illite and smectite are the most common clay minerals in the natural and man made depositories. They are also implicated in incomplete clarification of waste water treatment and soil remediation. Clays are sheet silicates platey in morphology and with high values of the ratio of platelet diameter to thickness, normally called the aspect ratio. Smectite’s unusual macroscopic properties are dominated by its structural arrangement and the morphology of its finest fraction. These clays are extremely dispersed and exhibit very high surface area of several hundred square meters per gram.
When clays have to be studied using oriented sample XRD and AFM imaging and force measurement methods, the clay suspension has to be dried on top of the flat surface like polished silicon wafer. Present investigations were inspired by search to find the best way of sample preparations to above listed investigations. During these investigations more interesting problems regarded structure forming phenomenon was encounter and resultant paper may contribute findings to discussion smectite behaviour in aqueous environment.
When a smectite powder is dispersed in water, the exchangeable cations diffuse into the dispersion medium and form an electric double layer. The Gouy–Chapman–
Stern model of the diffuse double layer is used to describe clay mineral dispersions bechaviour [1, 2]. The electric charge which occurs near clay-water interface triggers force driving particles within suspension and shaping structural type of particles mutual orientation. This orientation, when system gels are responsible for the type of microstructural type within any resultant clay deposit. The first attempt to describe the microstructure of clays was made by Terzaghi [3], who proposed the honeycomb model as the structural basis of water saturated clays. Subsequent investigations based on the development of new techniques in electron-microscopes [4] confirm the existence of the “card house” structure. Rosenquist [5], Bowles [6]. Pusch [7]
confirmed the presence of the honeycomb microstructure in wet clay sediments. Cryo- SEM investigation in O’Brien [8] published a large amount of microstructural data.
Given the size of the clay constituents, SEM was found to be the tool of choice used by scientists studying the microstructure of smectitic clays [9]. Most recently Synchrotron based Transmission X-Ray Microscopy (TXM) allowing 3-D
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examination of examined structure in stereo pairs as well as in computer calculated 3- D reconstruction [10, 11].
According to the DLVO theory, when electrostatic repulsive forces in the diffuse double layer keep particles apart, clay platelets may stay in stable suspension. When this force and diffuse layer extent is reduced by high salt concentration in solution or by lowering pH, the particles may approach each other and may form aggregates which will destabilise the suspension and form a gel. This explanation theory is only valid in highly dilute suspensions where particles are initially separated. In denser suspension, the closest particles interact with each other and start to form a three- dimensional expanded cellular structure as attraction between edge particle surfaces occurs concurrently with basal plane surface repulsion.
Experimental methods
The smectite used in this study was a well known Na-montmorillonite from Wyoming (U.S.A.) obtained from Clay Mineral Repository. This sample (SWy-2) has been well described [13] and is a smectite rich sample originating from postglacial natural shists deposits. Measurements were performed in distilled water. It should be noted that it is difficult to control Debye length in “water” because there is always some low level, 0.01 mM or less, of background electrolyte (including ions form the self-dissociation of water) that is hard to quantify or control. The aqueous electrolyte used for the suspension was MilliQ-water at natural pH. The sample slurry was disaggregated using an ultrasonic probe (Branson Sonic Power Company). The probe was immersed in 20 ml of the clay suspension sample and sonicated for ~45 seconds with 50 W energy applied. The sample container was rotated to assist in even dispersion and to aid deflocculation during sonication.
The studied clay sample structures were dried onto silicon wafer substrate. To prepare the different size fractions, the ultrasound pre-treated samples were agitated by shaking for ~30 seconds and left to settle for 5 minutes. This allows for settling of the >5 µm particle fraction with the <5 µm particle fraction found in the top 5 mm of the suspension. Using a Pasteur pipette, ~1.5 ml of sample slurry was removed from just below the meniscus (for the <5 µm particle fraction ) or deeper (for the >5 µm
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particle fraction ) and colloidal particles after centrifugation in 4000 revolution during 30 minutes. Suspension samples were applied to the silicon wafer 15mm x 25mm x 0.6mm plate, used as substrate and dried in air at room temperature.
Preferred orientation of clay platelets on a silicon wafer enhances the basal reflections and depress hk bands. Optimal flat orientation increases the basal (001) spacing to hk diffraction peak ratios. For smectite, the ratio of the basal reflection peak intensities (001) (d = 1.29 nm) to hk band peak intensities (020) (d = 0.45 nm) was also chosen after consideration of the sets of XRD patterns.
The force measurements was conducted using a Nanoscope III AFM (Digital Instruments, Santa Barbara) was used in the force mode with scan head E rate between 0.3 and 3 µm/s. We made our measurements on 200-µm long - wide V- shaped Si3N4 colloidal probe cantilevers as stated above. The spring constant of cantilever was 0.12 N/m. The force resolution on AFM is about 1 – 0.1 nN with the separation distance resolution is approximately 0.01 nm. The clay coated flat substrate surface was displaced in a controlled manner towards and away from the clay topped probe in aqueous solutions. The interacting forces between the probe and the flat surface can be obtained from the deflection of the cantilever. The deflection of the cantilever versus the displacement of the flat surface were converted into surface force versus separation by assuming that the zero point of separation was defined as the compliance region where the probe and the flat surface are in contact and the zero force is determined at large surface separations. The force measurements have been performed at a scan rate of 1 Hz over a scanning distance from 200 to 500 nm. Force measurements were performed in demineralised water further purified by a Milli-Q filtration system and in 0.01 and 0.1M NaCl solutions. Salts used in this work were analytical grade.
A FFI Quanta 200 Environmental SEM has been used without sample coating prior to observing particle orientation and distribution on the silicon wafer substrate.
A low voltage beam (5 kV) was used to minimise surface charging. A Panalytical X’Pert PRO XRD unit was used with Cu Kα at 40 keV, 40 mA. Samples were scanned in duration 12 min in the 2θ interval 3-35º using a post-diffraction monochromator and a multi-wire detector.
6 Results and Discussion
Before placement onto the silicon wafer for drying, the suspension was vitrified for cryo-SEM study. As shown in Fig. 1 the suspension resembled cellular structure where mineral flakes building elongated cells about 2 µm apart.
Fig. 1 Cryo-SEM micrograph Cellular micro-structure in gelled smectite 2 wt%
water suspension.
As observed in this cryo-SEM micrograph and previously reported in publications [12, 13], the basal particle surfaces appear to repel each other causing an expanded, spongy textural pattern consisting of aggregated individual particle platelets. Most platelets are arranged in a combination of folded sandwiched FF stacks which build elongated cells wall. Some of them contacted at the stack edges are connected in EF pattern making connection between elongated walls and building in consequence extended network through all of the area presented in this micrograph. This type of contact locks particles in random orientation in the gelled suspension. It may be expected that some elements of this orientation might be preserved in the sediment layer after drying as shown in Fig. 2.
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As it is evident from SEM micrographs in Fig 2, the surface of the smectite layer is not ideally smooth on the silicon wafer. It shows distinctive wrinkling which are less pronounced in the dry layer prepared from low density suspension (0.2 wt%) and very distinctive in the dry layer prepared from high concentration (2 wt%) smectite suspension.
Fig. 2- SEM micrograph, smectite low (A) and high (B) density suspension (0.2 - 2 wt%) in natural pH, fraction below 1 µm, dried on silicon wafer.
A
B
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We may interpret formation of wrinkles as the result of collapsing the 3-D structure during drying. When walls of the cellular structure like seen in Fig. 1 with parallel oriented to a silicon wafer collapsing in result of water been evaporated from within cells they may form flat lying smectite sheets. However platelets forming perpendicular walls will collapse more or less oriented texture towards a silicon wafer substrate. This effect intensified by strong shrinkage is observed when smectite is drying from highly dense (2 wt%) suspension like shown in Fig. 2B. Presence of large amount of randomly oriented wrinkles may have depressing effect on 001 peak
intensity and increasing 020 peak intensity which may shown differences in 001/020 peak ratios in samples prepared from suspensions of different densities. The XRD patterns presented in Fig. 3 give a 001/020 intensity ratio from the lower solid loading (0.2 wt%) 46.9 when ratio of the high solid loading suspension (2 wt%) is 23.6 which is twice lower than measured in clay film prepared from low density suspension. Than lower suspension density may influence better parallel platelet orientation in the dry sample on top of the flat surface of silicon wafer.
Fig. 3- XRD patterns of thin (0.2 wt%) and thick (2 wt%) smectite suspension clay deposited on a slide of the silicon wafer.
Force measurements
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For force measurements between smectite samples the dry samples were redispersed in water and with aid of ultrasonic bath, the dispersion of the clay samples was prepared from 0.2 wt% (thin film) of the dry mass of size fraction below 1 µm as recommended from findings in previous experience. A micro-drop of suspension was placed on the probe (Fig. 4) and few drops on substrate (silicon wafer) to produce clay topped surface, followed by drying in a laminar flow cabinet at room temperature overnight. Both surfaces were heated at a temperature of 80oC to firmly stick the clay flakes onto sphere and substrate surfaces.
Fig. 4- SEM micrograph with colloidal probe 2.5 µm in diameter at the tip of AFM silicon nitride cantilever.
AFM investigation on the oriented clay layers on top of the silicon wafer was chosen because it is very hard to locate the individual clay platelet and position the colloidal probe directly over it. In part this is due to the low refractive index difference between different clay crystals and water leading to low optical contrast and visibility of the platelet under water, and in part it is due to the mechanical arrangement of the AFM.
The thickness of the clay film was not measured but samples were prepared as thin layer (on silicon wafer) as specified in method above. In presented force – separation curves a long-range repulsion has been recorded whose range decreases as salt concentration increases. This is the signature of double-layer repulsion. Its presence indicates that, the clay/water interface must be negatively charged.
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According to the Poisson and Boltzmann's equations, one of the most characteristic parameters is what is called the Debye length which represents the thickness of the double layer. At constant temperature and dielectric permittivity, the thickness of the diffuse double layer depends only on the ionic strength.
The results of the test force measurements were performed using spherical bare silica glass probe against mica surface in water and 0.01 M and 0.1 M NaCl solutions.
In result of these measurements a rough comparison can be drown what thickness of the double layer can be detected as repulsive electrostatic force in solution of certain ionic strength. Test results were plotted with diagrams shown in Figure 5, in which the measured force between spherical probe (Fig. 4) and SWy-2 smectite colloidal film deposited on the silicon wafer on approach. Force F (normalised by the spherical probe’s radius of curvature R) is plotted on a logarithmic scale against the minimum distance D from the sphere’s surface to the flat plate. This is the standard way to plot surface force measurements, since the quantity F/R should be independent of the sphere’s radius and so data from different experiments can be compared. According to the Derjaguin approximation, F/R is 2π times the interaction energy between two parallel flat plates at the same separation D, and this quantity can easily be compared to theoretical calculations [14]. Positive values of F/R represent repulsion between the surfaces.
It is significant that the result of substitution and vacancies in the smectite lattice result in net charge about 0.66 – per unit cell and negative electric charge in aqueous solutions. This unit charge in micas may be twice larger 1.30 to 1.50, than electrostatic repulsion in case of smectite may be half of these measured and theoretically predicted in micas. However because of the atomically flat surface in mica, it is perfect object of the AFM test measurements and suitable for comparison to other complex substrates.
As the test, forces between bare silica sphere and mica surface was measured and compared with theoretical date from double layer calculation (Fig. 5). The test data show (placed as dashed liner in force curves Fig. 5 left column), a long-range electrostatic repulsion between these two surfaces, the quasi-exponential decay of the force, the decay length decreases as salt concentration increases and there is no adhesion between these surfaces. These results are entirely consistent with previous force measurements between silica and mica [15]. The repulsive force is explained by
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electrical double-layer repulsion between mice and silica probe surfaces which become charged on immersion in water. This leads to a quasi-exponential force whose range decreases with electrolyte concentration [14, 16]. Theoretically, a van der Waals attraction is expected at small separations, but this is not usually observed between silica and mica, probably due to the presence of short-range hydration repulsion [16].
In the case of studied smectite clay sample, deposited on silicon wafer, repulsion force in water of long range has been recorded when approach (Fig. 5A) from
relatively large distance of separation ~1000 nm. The shape of force curves presented in Fig. 5 is ramp-like and different from electrostatic interaction which should be measured in much shorter distances. Correct electrostatic interaction measured as repulsion forces between spherical probe and mica surface (dashed line) were plotted as the dashed lines to AFM results (when approach). These long distance structural forces may contribute to “steric” effect caused by restoration highly porous gelled cellular structure near the substrate surface like seen in Fig. 1 and presence of nano- particles which stretch far out of the clay interface into the bulk solution. Such structural effect may cushion impact of electrostatic forces which should be much stronger and could effect as the repulsive force much further out into solution. This effect was recently measured and was described in [17, 18]. Forces on the distance of separation ~1000 nm increases exponentially from 0.01 to 0.1 mN/m in water. This is not typical pattern of electrostatic repulsion which should be acting in a quarter shorter distances (250 nm).
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Fig. 5 Force – separation curves on approach (left column) and on retrieval (right column), for the interaction between; platelets in smectite clay sample (below 5 µm darker lines, colloidal fraction lighter lines), In water – (A), in 0.01 M NaCl (B), in 0.1 M NaCl (C). Dashed lines in the force measurement on approach represent forces measured between silica probe and bare mica surface in relevant NaCl electrolyte concentration.
Electrolyte addition influence significant reduction of force over similar distances of separation between silica probe and mica (dashed line in graphs Fig. 5B&C) which is strong evidence of the electric double layer interaction in this case. In smectite studied in NaCl electrolyte all force measurements were independent of electrolyte strength and all three concentrations congregated within the same zone regardless of electrolyte concentration and particle fraction (below 5 µm and colloidal fraction). In electrolyte concentration 0.1 M NaCl repulsive forces were recorded from similar distance of the clay surfaces separation as it was in case of distilled water.
The curves recorded on surfaces separation on retrieval in the smectite clay sample (Fig. 5 left column), qualitatively it is visible that strong adhesion was observed, i.e. the AFM probe after sinking into the cellular structure of clay sponge near the substrate interface was encounter strong adhesion when was pulled back on retraction. Probably structural elements from gelled clay layer in substrate surface and similar in spherical probe formed structural unity. Now this structure has to be
C
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destroyed AFM probe will reverse. In average, force curves on retrieval were larger to these recorded when colloidal probe approached clay surface. This is typical picture of hydrophilic interaction where similar structural interaction (by bubbles meniscus) which largely prevails electrostatic repulsion of electric double layer.
It is highly possible that these long-distance forces observed in water, similar but weaker to electrostatic repulsion, have structural origin. It may reflect the flexibility of the cushion-like cellular gel texture build of smectite flakes after immersion in water. In effect, hindrance of smectite gelled spongy structure build of the flexible platelets in clay film on AFM probe against the interaction between similar structures in substrate surface concerned platelets would be unavoidable. If this mechanism is responsible for our force measurement results it reflects squizzing cellular monolayer of the smectite gelled structure, which has dimensions that are consistent with
microscopy observations (Fig. 1).
One of the possible explanations is the heterogeneity of sizes of clays particles.
This is the point of a recent paper [23 and 24, 25] dealing with surface area of montmorillonite, and showing the influence, which is rarely taken into account of modification of platelets during adsorption of water.
In [25] stated that location of isomorphous substitution in the tetrahedral sheet of smectites results in an increased lateral extension of overlapping layers. This was reflected in a greater capacity to rehydrate after desiccation. Increased number of layers in particles was found with increasing surface charge density. The geometric organization of the particles is critical to the understanding of the ability of Na+
smectite to hold water against an applied suction.
However these articles dealing with montmorillonite structure changes during water adsorption in rather low wapour pressure. In present paper we studied structure modification entirely under water where montmorillonite flakes not only swelling but disintegrating into almost singular unit thick and form complex gelled structure on clay film and water interface which may be approximately up to 1.5 µm thick.
Clay minerals also are often accompanied by nano-clays. These mysterious particles being investigated only recently becoming a new subject in clay science [19, 20, 21] and have a lot of potential commercial applications. Nano-clay particles, because of their very small dimensions were overlooked in many previous studies but
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their impact on clay structure building behaviour is very important. In freeze-drying SEM image in Fig. 6A it is visible that chains of nano-particles seem to extending from larger clay particle aggregate and may bridge another aggregate or platelets with similar association of satellite nano-particles. Similar particles may be extended between clay surface and the clay coated colloidal probe during the AFM investigation influencing the long range interaction measured in clay systems especially in water and much diluted aqueous electrolytes. In the AFM micrograph shown in Fig. 6B nano-particles are associated with small (~1 µm in diameter) kaolinite platelet. It looks that this kaolinite crystal peeling off many of these nano- clays, ~1 nm thick and 10-50 nm in diameter, probably as an effect of chemical alteration at the mineral surface.
Fig. 7 The nanoclay particles (A)- SEM micrograph, extending from clay
aggregate and bridging other clay aggregates. (B)- AFM micrograph, scaling of the altered kaolinite surface.
Investigation of the nano-clays role in structure building phenomenon is not yet known and worth to devote more attention in future clay materials development and clay systems bechaviour.
Conclusions
Smectite drying phenomenon was investigated for purpose to gather knowledge for sample preparation for AFM and XRD investigations. Smectite in water
suspension display cellular 3-D structure with cells walls 1-2 µm apart. In
consequence of such structure building phenomenon smectite suspensions cannot form ideally flat surfaces when drying because the perpendicular cell walls influence wrinkling on the surface of drying substrate. Thicker smectite suspension, like 2 wt%
A B
1 µm
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because of multilayer cellular texture in addition to wrinkling structures, displays additional shrinkage and forms distinctive morphological pimply features. Uneven surface of dried smectite film can be monitored using 001/hk pick intensity ratio where larger contribution from 020 structural planes may be visible in disordered texture of dried clay film from thicker suspension.
The initial experiments described here have produced clear qualitative information about the interaction between smectite in water and NaCl salt solutions of 0.01M and 0.1M ionic strength. Repulsive double-layer forces were measured between all surfaces studied, which, demonstrates that the smectite are also negatively charged in water. Exponential nature of force increase in function of distances in clay sample shows electrostatic-like repulsion of electric double layers.
In smectite studied in NaCl electrolyte all force measurements were independent of electrolyte strength and size fraction. In graphs presented in Fig. 5 all three concentrations congregated within the same zone regardless of electrolyte
concentration and particle fraction (below 5 µm and colloidal fraction). Measured long-range repulsive forces were similar in range in water suspension as well as in NaCl electrolytes 0.01 and 0.1 M indicate that measured repulsion sensed in four times in larger distances than it may been calculated from Debey’ length and measured against mica surface may have different than electrostatic origin. Authors suggest that in water and water electrolytes, smectite surface layers forms gelled micro-structure when rehydrated which is spongy and cellular in texture and in consequence it cushion approaching AFM probe. This structural effect can be measured in distances larger than 1000 nm from substrate surface and when probe penetrate this gelled layer, structural linkages forming between substrate and clay covered probe. These linkages prevent subsequently smooth detachments of AFM probe on way back when retrieval. This effect of tearing new formed gel structure which bridging probe to substrate involves larger adhesion-like forces measured in retrieval. This effect may be similar to hydrophobic behaviour observed when
substrate and probe were linked by microscopically air bubbles. It is possible that this effect may be enhanced by the nano-clay particles interaction.
16 References:
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[9] Grabowska-Olszewska, B.; Osipov, V.; Sokolov, Vi. Atlas of the Microstructure of Clay Soils. PWN, Warszawa, 1984.
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[12] Van Olphen, H., Fripiat J.J. Data handbook for clay materials and other non- metallic minerals. Oxford; New York. Pergamon Press. 183, 1979.
[13] Smart, R.St.C.; śbik, M.; Morris, G.E. in (J.S. Laskowski Edt.) 43rd Annual Conference of Metallurgists of CIM. 2004, 215-228.
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List of Figures
Fig. 1 Cryo-SEM micrograph Cellular micro-structure in gelled smectite 2 wt%
water suspension.
Fig. 2- SEM micrograph, smectite low (A) and high (B) density suspension (0.2 - 2 wt%) in natural pH, fraction below 1 µm, dried on silicon wafer.
Fig. 3- XRD patterns of thin (0.2 wt%) and thick (2 wt%) smectite suspension clay deposited on a slide of the silicon wafer.
Fig. 4- SEM micrograph with colloidal probe 2.5 um in diameter at the tip of AFM silicon nitride cantilever.
Fig. 5 Force – separation curves on approach (left column) and on retrieval (right column), for the interaction between; platelets in smectite clay sample (below 5 µm darker lines, colloidal fraction lighter lines), In water – (A), in 0.01 M NaCl (B), in 0.1 M NaCl (C). Dashed lines in the force measurement on approach represent forces measured between silica probe and bare mica surface in relevant NaCl electrolyte concentration.
Fig. 6 The nanoclay particles (A)- SEM micrograph, extending from clay
aggregate and bridging other clay aggregates. (B)- AFM micrograph, scaling of the altered kaolinite surface.
18 Fig. 1
19 Fig. 2
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FIG. 5
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23 Fig. 6.
